Run the signing-parity check in CI

Run the signing-parity check in CI The vectors and the PHP suite were only a guard if something ran them, and nothing did: CI has no PHP job, so a change to the delivery token's canonical form would have passed every check and broken image rendering on every site using the connector — discovered by a customer, which is the outcome the fixture exists to prevent. A fixture nothing runs is decoration. The job does two things. It regenerates the vectors and diffs them against the committed file, so a Rust-side format change fails with a visible diff rather than quietly rewriting the file it is supposed to be checked against. Then it runs the PHP suite against those vectors. PHPUnit comes from the phar and there is no composer install. The module requires drupal/core, and pulling that in to assert a byte comparison would make the job guarding the wire format the slowest in CI — and a slow guard is one somebody eventually marks continue-on-error. The signer needs nothing from Drupal, which is the same property that lets it run in the render path, so tests/bootstrap.php autoloads the two classes directly. Both halves verified locally with the exact CI commands: the staleness check was confirmed to fail when the Rust output drifts, and the suite passes on a bare php:8.3-cli with no Drupal present.